In recent years, systems that record, transmit, and reproduce spatial information from the entire circumference have been developed and popularized in the field of sound. For example, broadcasting with three-dimensional multi-channel sound of 22.2 channels is planned in Super Hi-Vision.
Furthermore, systems that reproduce signals of sound surrounding the entire circumference in addition to video surrounding the entire circumference are also started to be distributed in the field of virtual reality.
Among these, there is so-called Ambisonics that is an expression method of three-dimensional sound information which can flexibly correspond to an arbitrary recording and reproduction system, and Ambisonics is drawing attention. Particularly, Ambisonics with second or higher order is called higher order Ambisonics (HOA) (for example, see NPL 1).
In the three-dimensional multi-channel sound, the information of the sound spreads to the spatial axis in addition to the temporal axis. In Ambisonics, frequency conversion, that is, spherical harmonic function conversion, is applied to the angle direction of the three-dimensional polar coordinates, and the information is held. The spherical harmonic function conversion can be considered to be equivalent to time-frequency conversion of the temporal axis of the sound signal.
An advantage of the method is that information can be encoded and decoded from an arbitrary microphone array to an arbitrary speaker array without limiting the number of microphones or the number of speakers.
On the other hand, factors that prevent popularization of Ambisonics include that a speaker array including a large number of speakers is necessary in the reproduction environment and that the reproduction range (sweet spot) of the sound space is narrow.
For example, although a speaker array including a larger number of speakers is necessary to increase the spatial resolution of sound, it is unrealistic to create such a system at home or the like. Furthermore, the area that can reproduce the sound space is narrow in a space such as a movie theater, and it is difficult to provide a desired effect to the entire audience.